Preheat oven to 350 degrees. Place cupcake liners in cupcake tin.
Add cake mix, eggs, and rainbow sherbet into mixing bowl. Stir to combine. Add in the flour and give a few stirs until just mixed in.
Add cake batter to the cupcake liners using a ⅓ cup of batter, filling until the cupcake liners are about ⅔ full.
Bake for 15 minutes or until baked through, check with a toothpick. Let the cupcakes cool in the baking tin for 5 minutes then remove to wire rack to continue cooling.
For Frosting
Add all ingredients to large mixing bowl.
Mix on low speed for 2-3 minutes until ingredients are combined. Then mix on medium speed for an additional 3 minutes until frosting is peaking, meaning that it is holding it's shape.
Separate into three bowls and add a flavor packet to each bowl. Stir to completely combine the flavor into the frosting, should have a uniform color throughout.
Frost using all three frostings, tutorial at House of Yumm here. I used a Wilton 2D tip for these cupcakes.